Trucks and other heavy vehicles are equipped with a steer axle, which generally bears the load of the front portion of the vehicle. The load on the truck, insofar as it affects the load on the steer axle, can be measured with a parasitic sensor that mechanically mimics the miniscule motion of the steer axle under load, specifically, a slight bending or depression in the middle of the steer axle. Strain gauges bonded to the sensor are read by a related electronics system. Such a device is described in U.S. Pat. No. 4,042,049 issued to Reichow et al. (hereafter referred to as the '049 patent).
A deflection transducer is often used in such a manner to obtain a fast and accurate “on-board” reading of vehicle load weight. An on-board weighing system permits convenient measurement of load and vehicle weight at any time without the necessity of using a conventional scale, such as at the time of initial loading, when part of the original load is removed from the vehicle, or when a partial new load is added. An on-board system prevents accidental overloads and the possible fines and other inconveniences associated therewith, while insuring that the vehicle is loaded substantially to its permitted capacity whenever possible.
However, conventional onboard weighing systems using deflection sensors, such as the system shown in the '049 patent, are typically subject to inaccurate or fluctuating weight readings due to changes in the sensor influenced by the ambient temperature adjacent to the sensor. Such temperature changes are usually not indicative of vehicle load, but they do have an effect on the sensor output.
More generally, parasitic weighing systems, including the system shown in the '049 patent, function by mimicking the deflection of the structural member to which they are attached through a calibrated linear function. That structural member has its own thermal coefficient of expansion, which has a further effect on the sensor output that is not indicative of the load on the structural member.
While a zero-offset temperature-compensating resistor is sometimes included in the strain gauge circuitry on a deflection sensor, its compensating effect is accurate only for a single load on the deflection sensor. Typically this is the no-load case, hence the term zero-offset. However, changing the load on the sensor will introduce inaccuracies in the temperature compensation and, for a zero-offset temperature-compensated deflection sensor, the greater the load means the greater the temperature-induced inaccuracy.
Likewise, those who are expert in the art of strain gauge sensors may, with considerable effort, be able to calibrate an individual sensor for the full span of weight and a partial range of temperature, with −10° C. to 40° C. being the strain gauge industry's standard. This requires testing each sensor over its temperature and weight range and physically installing or adjusting one or more discrete temperature-compensating resistors to account for the individual sensor's variability. However, the time and expertise required as well as the variability of the result make this impractical for general usage.
Whereas a sensor-processing microcomputer may sense its local ambient temperature and apply a blanket compensation appropriate to a distributed network of deflection sensors, such sensors are likely to be operating in different thermal environments. This difference in local sensor temperature can be the cause of inaccuracies, especially at temperature extremes.
Such a sensor-processing microcomputer could employ temperature sensors adjacent to each of the deflection sensors, but the additional wiring, programming considerations, and complexity of installing such a system would make this commercially prohibitive.
U.S. Pat. No. 4,543,837 to Stern et al. describes a load cell temperature-compensating system in which a mechanical fixed reference is used, but this method is cumbersome and does not make use of any statistical compensation. Some companies produce temperature-compensated on-board truck scales, but such scales rely on “load cell” type of sensors, and none of them are known to employ the more easily installed “deflection” type of sensors.
Thus, there exists a need for an economically-produced deflection sensor system capable of operating in on-board vehicle environments with compensation for a widely-changing thermal environment, assuring full possible design accuracy.
There also exists a need for a deflection sensor system capable of operating in on-board vehicle environments in which the compensation for the sensor's thermal coefficient is specifically optimized but not necessarily matched to the thermal coefficient of expansion of the structural member to which it is attached.
Additionally, there exists a need for a statistically-determined temperature-compensated load sensor system in which the characteristics of a class of load sensors can be averaged, with the results used within an electronic controller to produce an output signal that reflects substantially only the characteristics of the load, and is not responsive to variations in temperature.
Also, there exists a need for a sensor that can be matched to an underlying structural member in such a way as to compensate for dimensional changes in that structural member that are unrelated to weight, such as thermal expansion in one or more axes, or changes in the material hardness and other qualities at temperature extremes, so that the sensor is responsive only to variations in the weight bearing on the underlying structural member.
Additionally, there exists at the point of manufacture, a need for the ability to compensate for lot-to-lot variations in regards to the electronic qualities of a class of sensors' electronic components, such as gain and offset, to create greater uniformity of response from sensor to sensor.